This invention relates generally to magnetic disk drives, more particularly to spin valve magnetoresistive (MR) read heads, and most particularly to methods and structures for providing a pinning mechanism for spin valve sensors while minimizing pulse amplitude asymmetry.
Magnetic disk drives are used to store and retrieve data for digital electronic apparatuses such as computers. In FIGS. 1A and 1B, a magnetic disk drive D of the prior art includes a sealed enclosure 1, a disk drive motor 2, a magnetic disk 3, supported for rotation by a spindle S1 of motor 2, an actuator 4 and an arm 5 attached to a spindle S2 of actuator 4. A suspension 6 is coupled at one end to the arm 5, and at its other end to a read/write head, or transducer 7. The transducer 7 is typically an inductive write element with a sensor read element. As the motor 2 rotates the disk 3, as indicated by the arrow R, an air bearing is formed under the transducer 7 to lift it slightly off of the surface of the disk 3. Various magnetic xe2x80x9ctracksxe2x80x9d of information can be read from the magnetic disk 3 as the actuator 4 is caused to pivot in a short arc, as indicated by the arrows P. The design and manufacture of magnetic disk drives is well known to those skilled in the art.
The most common type of sensor used in the transducer 7 is the magnetoresistive (MR) sensor. An MR sensor is used to detect magnetic field signals by means of a changing resistance in a read element. A conventional MR sensor utilizes the anisotropic magnetoresistive (AMR) effect for such detection, where the read element resistance varies in proportion to the square of the cosine of the angle between the magnetization in the read element and the direction of a sense current flowing through the read element. When there is relative motion between the AMR sensor and a magnetic medium (such as a disk surface), a magnetic field from the medium causes a change in the direction of magnetization in the read element, thereby causing a corresponding change in resistance of the read element. The change in resistance can be detected to recover the recorded data on the magnetic medium.
Another form of magnetoresistive effect is known as the giant magnetoresistive (GMR) effect. A GMR sensor resistance also varies with variation of an external magnetic field, although by a different mechanism than with an AMR sensor. Sensors using the GMR effect are particularly attractive due to their greater sensitivity and higher total range in resistance than that experienced with AMR sensors. One type of GMR sensor is known as a spin valve sensor. In a spin valve sensor, two ferromagnetic (FM) layers are separated by a layer of non-magnetic metal, such as copper. One of the ferromagnetic layers is a xe2x80x9cfree,xe2x80x9d or sensing, layer, with the magnetization generally free to rotate when exposed to external fields. In contrast, the other ferromagnetic layer is a xe2x80x9cpinnedxe2x80x9d layer whose magnetization is substantially fixed, or pinned, in a particular direction. In the prior art, this pinning has typically been achieved with an exchanged-coupled antiferromagnetic (AFM) layer located adjacent to the pinned layer.
More particularly, and with reference to FIG. 2, a shielded, single-element magnetoresistive head (MRH) 10 includes a first shield 12, a second shield 14, and a spin valve sensor 16 disposed within a gap (G) between shields 12 and 14. An air bearing surface ABS is defined by the MRH 10. The spin valve sensor can be centered in the gap G to avoid self-biasing effects. Lines of magnetic flux impinging upon the spin valve sensor create a detectable change in resistance. The design and manufacture of magnetoresistive heads, such as MRH 10, are well known to those skilled in the art.
In FIG. 3 a cross-sectional view taken along line 3xe2x80x943 of FIG. 2 (i.e., from the direction of the air bearing surface ABS) illustrates the structure of the spin valve sensor 16 of the prior art. The spin valve sensor 16 includes a free layer 18, a copper layer 20, a pinned layer 22, and an antiferromagnetic (AFM) layer 24. The spin valve sensor 16 is supported by an insulating substrate 17 and a buffer layer 19 which can perform as a seed layer for the formation of the free layer 18 during fabrication. Ferromagnetic end regions 21, which operate as a hard bias, abut the ends of the spin valve sensor 16 and provide stabilization of the free layer 18. Leads 25, typically made from gold or another low resistance material, bring the current to the spin valve sensor 16. A capping layer 27 is provided over the AFM layer 24. A current source 29 provides a current Ib to flow through the various layers of the sensor 16, and signal detection circuitry 31 detects changes in resistance of the sensor 16 as it encounters magnetic fields.
The free and pinned layers are typically made from a soft ferromagnetic material such as permalloy. As is well known to those skilled in the art, permalloy is a magnetic material nominally including 81% nickel (Ni) and 19% iron (Fe). The layer 20 is typically copper. The AFM layer 24 is used to set the magnetic direction of the pinned layer 22, as will be discussed in greater detail below.
The purpose of the pinned layer 22 will be discussed with reference to FIGS. 4 and 5. In FIG. 4, the free layer 18 can have an actual free magnetization direction 26, while the pinned layer 22 has a pinned magnetization 28. Absent the magnetostatic coupling of the pinned layer 22, the ferromagnetic exchange coupling through the copper layer 20, and absent the field generated by the sensing current IS, the free layer 18 may have an initial free magnetization 30. The actual free magnetization direction 26 is the sum of the initial free magnetization 30 and the magnetostatic coupling of the pinned layer 22, the ferromagnetic exchange coupling through the copper layer 20, and the field generated by the sensing current IS. As is known in the art, the magnetization direction of the free layer 18 is preferably variable in response to varying external fields, for example from a nearby magnetic medium.
As seen in FIG. 5 on the curve of resistance versus magnetic field of the spin valve sensor, the pinned magnetization 28 of the pinned layer 22 at a right angle to the initial free magnetization 30 of the free layer 18 biases the free element to a point 32 on the curve that is relatively linear, and which has a relatively large slope. Linearity is, of course, desirable to provide a linear response, and the relatively large slope is desirable in that it produces large resistance changes in response to the changes in the magnetic field.
The antiferromagnetic material of the AFM layer 24 is typically either a manganese (Mn) alloy such as iron-manganese (FeMn) or an oxide such as nickel-oxide (NiO). The AFM layer 24 prevents the magnetization of the pinned layer 22 from rotating under most operating conditions, with the result that only the magnetic moment of the free layer 18 can rotate in the presence of an external magnetic field.
The spin valve sensor that has the most linear response and the widest dynamic range is one in which the magnetization of the pinned ferromagnetic layer 22 is parallel to the signal field and the magnetization of the free layer 18 is perpendicular to the signal field. However, the use of the AFM layer 24 to pin the pinned layer 22 presents several problems. For one, the exchange field strength generated by the AFM is highly sensitive to temperature. As the temperature increases, the AFM xe2x80x9csoftensxe2x80x9d and its ability to fix the magnetization of the pinned ferromagnetic layer decreases. In consequence, spin valve sensors are highly sensitive to electrostatic discharge (ESD) currents and the resultant heating of the AFM layer 24. Further, AFM materials such as FeMn are much more susceptible to corrosion than the other materials used in the spin valve sensor. The sensitivity of the AFM materials requires careful control of the fabrication process steps and the use of the protective materials for the spin valve sensor. AFM films 24 are also difficult to manufacture, in that they may require high annealing temperatures to obtain the proper crystallographic antiferromagnetic phase.
The present invention eliminates the need for the antiferromagnetic (AFM) layer in a spin valve sensor, while doubling the read signal and operating in a highly symmetrical differential, or common reject mode, and therefor exhibits high read performance. This is accomplished by providing a pair of spin valve stripes which each include correctly configured free and pinned layers. Further, two separate currents are provided to the spin valve stripes, each of which thereby operates to substantially fix the magnetization of the respective pinned layer, while the combined magnetic effect of the two currents on the free layers is minimized. In addition, the read circuit is configured to combine the difference of the voltage signals from each spin valve stripe.
In an embodiment of the present invention, a dual-stripe current-pinned spin valve magnetoresistive read sensor includes a first soft ferromagnetic (FM) layer having a first Mrt and a second soft FM layer having a second Mrt greater than the first Mrt. A first spacer layer formed of conductive material is disposed between and separating the first and second soft FM layers. The first spacer layer is also configured to receive a first biasing current for generating a first magnetic field of sufficient strength, relative to the first Mrt, to saturate the first soft FM layer. The read sensor further includes a third soft FM layer having a third Mrt and a fourth soft FM layer having a fourth Mrt less than the third Mrt. A second spacer layer formed of conductive material, disposed between and separating the third and fourth soft FM layers, and configured to receive a second biasing current for generating a second magnetic field of sufficient strength, relative to the fourth Mrt, to saturate the fourth soft FM layer. An insulation layer also is disposed between the second soft FM layer and the third FM layer. Further, the first, second, third, and fourth soft FM layers are configured such that when the first and second magnetic fields have sufficient strength to saturate the first and fourth soft FM layers, respectively, the first and second magnetic fields have a combined negligible effect on magnetizations of substantial portions of the second and third soft ferromagnetic layers. By manipulating both of the two spin valve read signals, the overall response of the dual-stripe current-pinned spin valve read sensor is highly symmetrical, exhibits a large read signal, and operates in differential, or common reject mode, and therefor exhibits high read performance. Further, without an AFM layer, the manufacturing complexities and the temperature and ESD sensitivity of the sensor are reduced, while the reliability of the sensor is increased.
In another embodiment of the present invention, a read/write head for accessing and storing data on a medium includes an inductive write element and a dual-stripe current-pinned spin valve magnetoresistive read element. The dual-stripe current-pinned spin valve magnetoresistive read element includes a first soft ferromagnetic layer and a second soft ferromagnetic layer separated by a first spacer layer formed of conductive material. The read element also includes a first lead set formed of conductive material, electrically connected to the first spacer layer and configured to pass a first biasing current through the first spacer layer for pinning the first soft ferromagnetic layer. In addition, a third soft ferromagnetic layer and a fourth soft ferromagnetic layer separated by a second spacer layer formed of conductive material are included. A second lead set formed of conductive material is electrically connected to the second spacer layer and configured to receive a second biasing current for pinning the fourth soft ferromagnetic layer. Additionally, an insulation layer is disposed between the second soft ferromagnetic layer and the third ferromagnetic layer and is formed of electrically insulating material. More specifically, the first, second, third, and fourth soft ferromagnetic layers are configured such that when the first and second biasing currents pin the first and fourth soft ferromagnetic layers, the first and second biasing currents have a combined negligible effect on magnetizations of substantial portions of the second and third soft ferromagnetic layers. In this way, two layers are substantially pinned without the use of an AFM layer, while the magnetizations of two other layers remain substantially free. Exclusion of an AFM layer greatly reduces the manufacturing complexities, reduces the temperature and ESD sensitivity of the sensor, and increases the reliability of the sensor. Also, with the two sets of free and pinned layers, the read performance of the read/write head can be significantly enhanced due to increased signal, high symmetry, and dual-mode operation.
In yet another embodiment of the present invention, a method for forming a dual-stripe current-pinned spin valve read sensor includes forming a first soft ferromagnetic layer, forming a first spacer layer formed of a conductive material above the first ferromagnetic layer, and forming a second soft ferromagnetic layer above the first spacer layer. The method also includes forming an insulator layer of electrically insulating material above the second soft ferromagnetic layer. Above the insulator layer, a third soft ferromagnetic layer is formed, a second spacer layer is formed of a conductive material above the third soft ferromagnetic layer, and a fourth soft ferromagnetic layer is formed above the second spacer layer. Additionally, the method includes forming a first lead set electrically connected to the first spacer layer for passing a first biasing current through the first spacer layer to pin the first soft ferromagnetic layer, and forming a second lead set electrically connected to the second spacer layer for passing a second biasing current through the second spacer to pin the fourth soft ferromagnetic layer. Further, the first, second, third, and fourth soft ferromagnetic layers are formed such that when the first and second biasing currents pin the first and fourth soft ferromagnetic layers, the first and second biasing currents have a combined negligible effect on magnetizations of substantial portions of the second and third soft ferromagnetic layers. Thus, this method provides a read sensor that includes well pinned layers, without the use of an AFM layer which could impose limitations on the fabrication and use of the read sensor. Further, with this method a read sensor is formed that provides a high read signal with high symmetry, and operates in dual-mode to provide high read performance.
These and other advantages of the present invention will become apparent to those skilled in the art upon a reading of the following descriptions of the invention and a study of the several figures of the drawing.